Seasonality of infectious disease in Åland, Finland

Interest in seasonal patterns of infectious diseases is increasingly important given recent debates over the ramifications of global warming. Diseases are dynamic, responding to variations in temperature, rainfall, abundance of hosts, parasite biology, and resource availability. Seasonality also impacts geographic variation in onset, persistence, timing, and severity of epidemics. Hence, studying seasonality over a lengthy period provides a temporal reference useful for understanding and predicting how environmental trends will impact health in the future.

This study examines the effect of infectious diseases on populations residing in the Åland Islands, Finland, from 1650 to 1950. Primary data sources used are parish records of deaths and births kept by Åland's Lutheran ministers. The data consist of over 75,000 incidences of death. Environmental data consist of average monthly minimum and maximum temperatures, the amount of rainfall and number of wet days per month. The data are compared to seasonal variation during “normal” years for all deaths, not just those of infectious origin.

The results reveal a definite sinusoidal curve for some infectious diseases that are similar to the curve for total deaths. On the other hand, measles deaths display distinct seasonal patterns. Statistical analyses reveal infectious disease deaths correlate significantly with the seasonal pattern of total deaths, but these infectious diseases remain unique in their means and distributions. Further analysis identifies significant relationships between temperature and rainfall with total deaths and deaths from infectious disease. These patterns are discussed in relationship to seasonal patterns found in other populations and their driving environmental factors.

Research supported by a GRA position in the College of Liberal Arts and Sciences at the University of Kansas.